Process for the production of hydrogen peroxide

ABSTRACT

The invention relates to a process for the production of hydrogen peroxide by the anthraquinone process, comprising a hydrogenation stage, an oxidation stage and an extraction stage. According to the invention, catalytic hydrogenation of anthraquinone derivatives dissolved in a working solution is carried out in the presence of added molecular oxygen. Per mol hydrogen, 0.1 to 10 mmol oxygen is preferably introduced into the hydrogenation stage with the hydrogenating gas, in mixture with an inert gas and/or dissolved and/or dispersed in the working solution. This increases the residence time of the catalyst.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application represents U.S. national stage of internationalapplication PCT/EP03/02181, which had an international filing date ofMar. 4, 2003 and which was published in English under PCT Article 21(2)on Oct. 9, 2003. The international application claims priority toapplication EP 02007120.5, filed with the European Patent Office on Mar.28, 2002.

DESCRIPTION

1. Field of the Invention

The invention relates to a process for the production of hydrogenperoxide by the anthraquinone process, by means of alternate reductionand oxidation of a working solution containing one or more anthraquinonederivatives. The invention produces an improvement in catalyst residencetime in the hydrogenation stage of the process.

2. Background of the Invention

The production of hydrogen peroxide by the so-called anthraquinoneprocess is known. This process is based on the alternate hydrogenationand oxidation of anthraquinone derivatives, conventionally2-alkylanthraquinones and 2-alkyltetrahydroanthraquinones, the alkylgroup being linear or branched and generally having 2 to 6 carbon atoms.The anthraquinone derivatives referred to, and the anthraquinonederivatives obtained in the hydrogenation stage will hereinafier beknown by the general name “reactants.” In the anthraquinone process,these reactants are dissolved in an organic solvent system, the solutionbeing known as a “working solution.” The working solution frequentlycontains two different 2-alkylanthraquinones and their tetrahydroderivatives.

In the hydrogenation stage of the anthraquinone process, thealkylanthraquinones and alkyl tetrahydroanthraquinones contained in theworking solution are at least partially converted with hydrogen or ahydrogen-containing gas in the presence of a catalyst into thecorresponding alkylanthrahydroquinones or alkyltetrahydroanthraquinones.The working solution is hydrogenated in the presence of a suspensioncatalyst, in particular a precious metal-containing suspension catalyst.Alternatively, hydrogenation of the working solution in the presence ofa fixed bed catalyst arranged in a hydrogenating reactor is also known.

The hydrogenated working solution freed from catalyst is then treatedwith an oxygen-containing gas, conventionally with air, thealkylanthraquinones or alkyltetrahydroanthraquinones being re-convertedand hydrogen peroxide being formed at the same time.

Hydrogen peroxide is then isolated from the oxidised working solution.This is mostly an extraction step, hydrogen peroxide being extractedwith an aqueous solution and this solution then being purified andconcentrated. The working solution is returned to the hydrogenationstage. The anthraquinone process generally also includes process stagesfrom the working solution regeneration and catalyst regeneration series.A summary of the anthraquinone process for the production of hydrogenperoxide and various embodiments of individual stages can be found inUllmann's Encyclopedia of Ind. Chem., 5^(th) ed., Vol. A 13, P. 447–456.

The activity, productivity and residence time of the hydrogenatingcatalyst and the selectivity with which it hydrogenates theanthraquinone derivatives are crucial to the economy of theanthraquinone process.

When using suspension catalysts, such as palladium black or supportedpalladium catalysts for example, the catalyst de-activation in thehydrogenation stage is taken into account by periodically orcontinuously separating part of the catalyst from the working solution,regenerating it outside of the actual process and returning it to thehydrogenation stage. Although this catalyst separation and externalregeneration is technically expensive, it avoids the reduction incapacity that results when a plant is shut down for catalystregeneration.

The use of fixed bed catalysts in the hydrogenation stage is also known:

In the process according to U.S. Pat. No. 3,009,782 a fixed bed withcatalyst particles bound to a support is used in the hydrogenation stageand as a result, fewer by-products are formed and selectivity is therebyincreased.

Although the monolithic honeycomb-shaped fixed bed catalyst used in thehydrogenation stage of the process according to U.S. Pat. No. 4,552,748and the static mixer coated with catalyst used according to U.S. Pat.No. 5,071,634 produce an acceptable catalyst residence time, here toothe fixed bed reactor must be regenerated periodically to increaseactivity again.

Finally, in the process according to DE 19953185 the catalyst residencetime of a fixed bed reactor filled with a catalyst in particle form isincreased by operating the fixed bed reactor as an upward current bubblecolumn.

A disadvantage of all of the variants of fixed bed hydrogenationdescribed by way of example above, is that in spite of improved catalystresidence times, the de-activated catalyst must periodically beregenerated. The catalyst is regenerated or re-processed either byremoving the catalyst from the reactor for external processing or, noless expensively, by shutting down the reactor and regenerating thede-activated catalyst without removing it. Interrupting production forregeneration can be avoided if several hydrogenating reactors areinstalled, each of which is used in turn for hydrogenation and forregeneration. However this increases the investment costs of such aproduction system and reduces its economy.

Attempts were made to reduce the problem of limited catalyst residencetime by selecting special catalyst recipes with a low de-activationrate. Thus U.S. Pat. No. 4,800,075 discloses the use of a palladiumcatalyst on an alpha aluminium oxide support with a BET specific surfacearea of 5–108 m2/g and DE-OS 19713376 discloses the use of a palladiumcatalyst on a silica support with an average pore diameter of 80–400Angstrom. However, even here, provision must be made in an industrialproduction plant for catalyst regeneration.

According to EP 0778085 A1, a hydrogenating catalyst can be regeneratedand activated by treatment of the catalyst with an acid. However, thisregeneration is either carried out externally or requires operation tobe interrupted and also leads to the use of alien chemicals.

In the process according to EP 0670182 A1, according to which thede-activated hydrogenating catalyst is regenerated in the hydrogenatingreactor by bringing it into contact for several hours with oxidisedworking solution, the hydrogen feed and thus hydrogenation must also besuspended.

In the process according to U.S. Pat. No. 3,004,831 the de-activatedcatalyst is regenerated by periodically reducing the hydrogen pressurein the hydrogenation stage and passing an inert gas through the reactorfor a sufficient period of time. This process thus requires operation tobe suspended.

Only in the process according to DE-OS 20 42 523 can the selectivity andactivity of the hydrogenating catalyst be maintained over a long periodwithout interrupting the hydrogenation reaction, by means of in situregeneration which is carried out using a working solution to behydrogenated which contains at least 250 mg/l, in particular 300 to 1000mg H₂O₂/l reactive hydrogen peroxide. The required quantity of hydrogenperoxide can be set by partial extraction of the oxidised workingsolution or by adding an appropriate quantity of oxidised workingsolution to a fully-extracted working solution. The disadvantage of thisprocess is that feeding part of the previously-formed hydrogen peroxideinto the hydrogenation stage reduces the yield.

DESCRIPTION OF THE INVENTION

An object of the present invention is to demonstrate an improvement inthe hydrogenation stage of the anthraquinone process for the productionof hydrogen peroxide, which increases the catalyst residence timewithout suspending hydrogenation and without having to usepreviously-formed hydrogen peroxide.

A further object is that the process should be simple to carry out andshould require no alien auxiliary substances.

A further object is that the catalyst residence time should be increasedin particular in fixed bed reactors, including in a trickle bed reactoror a bubble column reactor, because with these embodiments the hydrogenperoxide taken in with the working solution is obviously not effectiveover the whole area of the reactor.

The objects mentioned above and other objects arising from thedescription that follows, can be achieved by the process according tothe invention, according to which the the working solution ishydrogenated in the presence of a small quantity of added elementaloxygen.

A process for the production of hydrogen peroxide by the anthraquinoneprocess was found, comprising a hydrogenation stage, in whichanthraquinone derivatives contained in a working solution arehydrogenated with a hydrogen-containing gas in the presence of ahydrogenating catalyst, an oxidation stage, in which theanthrahydroquinone derivatives formed in the hydrogenation stage arere-converted into anthraquinone derivatives with an oxygen-containinggas, in particular air, with the formation of hydrogen peroxide,isolation of the hydrogen peroxide from the oxidised working solutionand return of the working solution to the hydrogenation stage, which ischaracterised in that hydrogenation is carried out in the presence ofadded molecular oxygen (O₂), in a quantity of at least 0.02 mmol O₂ permol H₂ which is below the explosion limit under the hydrogenationconditions and which is introduced into the hydrogenaton stage in theform of an oxygen-containing gas. The sub-claims relate to preferredembodiments of the process according to the invention.

Molecular oxygen, or preferably air, is fed into the hydrogenatingreactor at one or more points, either directly or together with thehydrogen-containing hydrogenating gas and/or in mixture with an inertgas, such as further nitrogen, and/or with a working solution to behydrogenated, which has previously been charged with oxygen. The lattercan be obtained by bringing the extracted working solution into contactwith air or oxygen. When dosing the oxygen or air into the hydrogenatinggas or into the hydrogenating reactor, it must be ensured that nocritical explosion ranges are crossed, even locally. By purifyingpreviously-produced hydrogen accordingly, the quantity of oxygenrequired according to the invention can be incorporated into thehydrogen. The upper limit of the oxygen content is determined solely bysafety requirements (explosion limit); the minimum quantity is measuredin such a way as to bring about an effective increase in catalystresidence time. Surprisingly, by using the measures according to theinvention, the residence time of the catalyst is significantlyincreased.

The quantity of oxygen used is at least 0.02 mol O₂/mol H₂ and ispreferably in the range from 0.1 to 20 mmol O₂ per mol hydrogen,although the levels may be below or above these limits. In particular,the quantity used is preferably in the range from 0.5 to 10 mmol O₂ permol hydrogen. If the hydrogenating gas consists substantially ofhydrogen and the oxygen is introduced into the hydrogenating reactorwith the hydrogen, this mixture preferably contains 100 to 20,000 vpm O₂(vpm=volume parts per million). If oxygen is introduced into thehydrogenating reactor with the working solution to be hydrogenated, theoxygen can be dissolved and/or finely dispersed therein.

Depending on the composition of the working solution, the extractionconditions and subsequent treatment, such as drying under reducedpressure, the working solution fed into the hydrogenating reactor canstill contain a small quantity of hydrogen peroxide. It is assumed thatthis hydrogen peroxide can be decomposed to water and oxygen in thepresence of the hydrogenating catalyst and additionally exerts a certaininfluence on the increase of the catalyst residence time. By addingelemental oxygen according to the invention, in other words oxygen thatdoes not originate from the hydrogen peroxide still present in theworking solution, the catalyst residence time is further improved. Thiseffect is particularly clear if the hydrogenating reactor is a fixed bedformed from catalyst in particle form and the reactor is operated as abubble column or trickle bed.

Although R. Willstätter et. al. in Chemische Berichte 54B, 113–123,(1921) reported that the catalytic hydrogenation of anhydrides, aromaticacids and cinnamic acid esters with platinum or palladium catalysts isaccelerated in the presence of oxygen, oxygen has surprisingly neverbefore been introduced into the hydrogenation stage of the anthraquinoneprocess for the production of hydrogen peroxide.

The known reaction supports, catalysts and process variants ofhydrogenation as well as the known solvents for the working solution canbe used in the process according to the invention.

Particularly suitable reactants are 2-alkylanthraquinones and their corehydrogenated 2-alkyltetrahydroanthraquinones, the alkyl group having twoto six carbon atoms and being either linear or branched. A combinationcontaining 2-ethylanthraquinone and a C₄- to C₆-alkylanthraquinone andtheir tetrahydro derivatives is preferably used as the reactant. Theworking solution contains one or more solvents, in which both theanthraquinone derivatives and the anthrahydroquinone derivatives arehighly soluble.

The catalysts conventionally used in the anthraquinone process can beused in the process according to the invention. These are preferablyprecious metal-containing catlysts, in particular palladium-containingcatalysts. The catalytically-active component may be present in free orsupported form or as a constituent of a coating on a honeycomb reactoror on a static mixer.

Hydrogenation is carried out by the method known per se with regard totemperature and pressure and the flow ratios and thus contact time inthe reactor. The temperature is mostly in the range from 10 to 100° C.,in particular 40 to 80° C., the pressure in the range 0.01 to 2 mPa, inparticular 0.1 to 0.7 mPa. Hydrogenation can be carried out in knownreactors for suspension hydrogenation or fixed bed hydrogenation.

According to a preferred embodiment the dimensions of the reactor aresuch and the hydrogenation conditions are set in such a way that thehydrogen fed into the reactor is completely consumed on its way throughthe reactor.

Advantages of the process according to the invention are that theresidence time of the catalyst is significantly higher than that of thepreviously-known operating method, that there is no need for a costlyhydrogenation reactor construction, that the process can be used bothfor fixed bed and suspension hydrogenation, although fixed bedhydrogenation is particularly advantageous, and the hydrogenation stageis not bound to a particular composition of the working solution and/orhydrogenating catalyst.

The invention is further explained by the following examples (B) andreference examples (VB).

EXAMPLES

Hydrogenation was carried out continuously in a reaction tube with 5 mlcatalyst bulk volume. The reactor was 10 mm in diameter. The unitconsisted of a liquid receiver, the reactor and a liquid separator. Thereaction temperature was set by means of a heat exchanger-oilcirculation. The pressure and hydrogen stream were regulatedelectronically. The working solution was dosed into a hydrogen streamwith a pump and the mixture was released from the top of the reactor(trickle-bed method). After passing through the reactor, the product wasremoved from the separator at regular intervals.

The working solution, which was withdrawn from an anthraquinonecirculation process after the extraction stage, contained alkylaromatics and tetrabutylurea as solvents and a mixture of2-alkylanthraquinones and their 2-alkyltetrahydroanthraquinones in amolar ratio of 1:3.2 as reactants.

The reactor overpressure in the examples and reference examples was ineach case 0.2 mPa. The LHSV liquid charge was 10 h⁻¹ and the reactortemperature 76° C. in all cases. The hydrogen-containing gas stream fedinto the reactor was 4 Nl/h in all cases.

Hydrogen which, according to the manufacturer's information, containedoxygen in the range<10 vpm was used as hydrogenating gas in referenceexamples VB 1.1 to VB 1.5. Hydrogen enriched to an oxygen content of3000 vpm was used in the examples according to the invention B 1.1 to B1.5.

The catalyst used was a supported catalyst, namely palladium on SiO₂(Aerolyst, Degussa). The particle size distribution of the granularsupported catalyst was 1.0–1.4 mm.

An aqueous palladium nitrate solution was used to charge the support. 50g of the support material was placed into a coating pan and a solutionof 200 g water und 270 mg palladium nitrate was poured on whilst the panrotated. The coated support was air-dried at 170° C. for 12 h. Thecatalyst was then reduced in the reactor with hydrogen (<10 vpm O₂) at100° C. for 2 h.

Table 1 below shows the results of examples B 1.1 to B 1.5 according tothe invention and reference examples VB 1.1 to VB 1.5. TheH₂O₂-equivalent is given as a measure of hydrogenation as a function ofthe operating period.

TABLE Operating H₂O₂- period equivalent No. [h] [g/l] VB 1.1 1 9.1 VB1.2 21 4.3 VB 1.3 43 2.2 VB 1.4 120 0.7 VB 1.5 146 0.5 B 1.1 2 7.9 B 1.223 5.8 B 1.3 42 4.9 B 1.4 119 3.8 B 1.5 145 3.6

The tests show that for the operating period selected, the H₂O₂equivalent of the embodiment according to the invention remainsvirtually constant after an initial loss of activity. With thepreviously-known embodiment, in other words hydrogenation without aquantity of oxygen active according to the invention, the loss ofactivity is many times greater and no constant value is achieved, theloss of activity continuing until full de-activation.

Reference Examples 2.1 to 2.5

These examples were carried out in the same way as reference example 1,although a non-extracted oxidised working solution was added to theextracted working solution before the hydrogenation stage, so that themixture had an H₂O₂ equivalent in the range from 0.5 to 0.3 g/l. Theresults are shown in Table 2.

TABLE 2 H₂O₂ equivalent Operating H₂O₂ equivalent after period in thehydrogenation No. [h] receiver [g/l] [g/l] VB 2.1 1 0.5 9.7 VB 2.2 230.5 7.7 VB 2.3 47 0.4 6.3 VB 2.4 119 0.5 3.5 VB 2.5 143 0.3 2.8

These reference examples show that although the addition of H₂O₂ to theworking solution to be hydrogenated improves the residence time of thecatalyst, the effect is far less than that obtained by the processaccording to the invention and, once again, the previously-known processuses pre-formed h₂O₂.

1. In a process for the production of hydrogen peroxide, comprising: a)a hydrogenation stage reaction comprising the hydrogenation of ananthraquinone derivative with a hydrogen-containing gas in the presenceof a hydrogenating catalyst to form an anthrahydroquinone derivative,said anthraquinone derivative and anthrahydroquinone derivative beingcontained within a working solution; b) an oxidation stage reactioncomprising the reconversion of said anthrahydroquinone of step a) backto said anthraquinone derivative using an oxygen-containing gas, saidreconversion being accompanied by the formation of hydrogen peroxide;and c) an extraction stage comprising isolating said hydrogen peroxideformed in step b) and then returning the working solution to saidhydrogenation stage reaction, the improvement comprising addingmolecular oxygen (O₂) in the form of a gas containing molecular oxygenas a component to said hydrogenation stage reaction in a quantity of atleast 0.02 mmol O₂ per mole H₂ and less than the explosion limit underthe hydrogenation conditions.
 2. The process of claim 1, wherein saidmolecular oxygen is added to said hydrogenation step reaction in a formselected from the group consisting of: O₂ alone; air alone; O₂ or air inmixture with said hydrogen-containing gas of said hydrogenation stagereaction; and O₂ or air in mixture with an inert gas.
 3. The process ofclaim 1, wherein said molecular oxygen is added to said hydrogenationstage reaction in a quantity of 0.1 to 20 mmol O₂ per mole of hydrogen.4. The process of claim 3, wherein molecular oxygen is added in aquantity of 0.5 to 10 mmol of O₂ per mole of hydrogen.
 5. The process ofclaim 1, wherein said hydrogen-containing gas in said hydrogenationstage reaction has an oxygen content of 100 vpm (volume parts permillion) to 5000 vpm.
 6. The process of claim 1, wherein saidhydrogenation stage reaction is carried out in a fixed bed reactor withan LHSV (liquid hourly space velocity) of 0.1 h⁻¹ to 20 h⁻¹.
 7. Theprocess of claim 1, wherein a precious metal-containing fixed bedcatalyst of particles having an average diameter of 0.5–20 mm is usedfor said process.
 8. The process of claim 7, wherein said fixed bedcatalyst contains palladium.
 9. The process of claim 6, wherein saidfixed bed reactor is operated as a trickle bed.
 10. The process of anyone of claim 1–9, wherein, after the isolation of said hydrogen peroxidein said extraction stage, said working solution is brought into contactwith said molecular oxygen or gas containing molecular oxygen beforebeing returned to said hydrogen stage reaction.
 11. The process of anyone of claims 1–9, wherein said molecular oxygen or gas containingmolecular oxygen is added directly to a reactor in which saidhydrogenation stage reaction is taking place, said addition occurringafter the isolation of said hydrogen peroxide, and wherein said additionis made in a quantity of 0.1 to 20 mmol of O₂ per mole of hydrogen.